The present invention relates to an acousto-optical deflector. It is a component comprising essentially a crystal used as interaction medium between an ultrasonic wave and an incident light wave for diffracting and so deflecting this light wave. Such a component is sometimes designated under the name of Bragg cell. The applications of such a deflector are various. It can be used in the aerospatial field. It can also be used as a deflection mirror in laser printers.
An acousto-optical deflector comprises then more precisely a crystal and a piezoelectric transducer controlled by a radiofrequency (RF) generator, for generating an ultrasonic wave in the crystal and causing its diffraction index to vary.
An incident light beam then gives rise in the crystal to a beam diffracted in the plane of the acoustic waves and luminous, deflected through an angle depending on the frequency of the acoustic wave. A diffracted beam coming from an incident light beam, even if the latter is a parallel beam, cannot be focussed on a point. A beam diffracted by an acousto-optical deflector always diverges. It is focussed in a spot whose dimension is limited, downwards, by the diffraction itself.
In the application to printers, the following parameters are to be considered:
length of the printing line, PA0 dimension of the image element, or pixels, PA0 number of pixels per line or resolution, PA0 time for scanning a line. PA0 .phi. is the opening diameter of the light beam, PA0 .DELTA.F is the acoustic pass-band, and PA0 k a factor of proportionality. PA0 T is the line scan time corresponding to the duration of the frequency sweep from one end to the other of the acoustic spectrum.
The dimension of the pixels corresponds to the dimension of the spot limiting the diffracted beam. A line is scanned by varying the acoustic frequency. It must then be recalled that an incident beam only gives rise to a diffracted beam after the sound wave, in the crystal, has passed through the incident light beam. It is a question of the random access time which depends naturally on the opening of the incident light beam. Finally, the resolution of printers using an acousto-optical deflector is proportional to this beam opening and to the width of the spectrum of the acoustic frequencies, i.e. the acoustic pass-band.
Such reminders being given, it can be announced that the problem which the Applicant has sought to solve and which is at the origin of his invention is that of increasing the resolution of acousto-optical deflector printers without increasing the line scan time.